Fixing wait, exit, ptrace, exec, and CLONE_THREAD

All,

I am posting this patches in the hope of some review of the strategy I am taking and to let the individual patches be reviewed.

The intersection of wait, exit, ptrace, exec, and CLONE_THREAD does not work correctly in the linux kernel.

An easy entry point into seeing the problem is that exec can wait forever holding a mutex while waiting for userspace to call waitpid on a ptraced thread. This is bad enough it has been observed to cause deadlocks in real world situations today.

Another easy entry point is to see that a multi-threaded setuid won't change the credentials on a zombie thread group leader. Which can allow sending signals to a process that the credential change should forbid. This is in violation of posix and the semantics we attempt to enforce in linux.

I have been silent the last couple of weeks reading up on the history and seeing if I could come up with a solution to the buggy semantics and great big piles of technical debt that exists in this area of the kernel. It unfortunately requires understanding all of the pieces to come up with a satisfactory long term solution to this problem. My goal is to fix enough of the issues that futher cleanups and bug fixes won't require such comprehensive knowledge.

In large the solution I have found is to:

- Fix the semantics of wait.

This makes it clear what the semantics the rest of the fixes need to maintain.

- To move everything possible from release_task to do_exit.

Moving code from release_task to do_exit means that zombie threads can no longer be used to access shared thread group state, removing the problem of stale credentials on zombie threads.

Much of the state that is freed in release_task instead of do_exit is freed there because originally during the CLONE_THREAD development there was no where else to put code that freed thread group state. And my changes largely move state freeing back where it used to be in 2.4 and earlier.

- To remove the concept of thread group leader (and allowing the first thread to exit).

The concept of thread group leader keeps leading people into false mental models of what the kernel actually does.

There is a lot of code that makes the mistake of assuming the thread group leader is always alive. When the code doesn't make that mistake it requires additional code complexity to deal with zombie thread group leaders. As seen in the signal code which still doesn't handle zombie thread group leaders correctly from a permission checking perspective.

- To promote tgid to a first class concept in the kernel.

This is necessary if there isn't a thread group leader, and a significant part of the functional changes I am making.

- To only allow sending signals through living threads.

This is a subset of removing the concept of thread group leader. But worth calling out because signal handling is the most significant offender.

My first batch of changes includes some trivial cleanups and then includes the small semantic changes (AKA user visible) I noticed that are necessary to give userspace consistent semantics.

The full set of changes that I have come up with is at:
git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace.git exit-cleanups-for-testing

Unless someone has a better suggestion I intend to collect this up as a topic branch and merge this through my tree.

Review on the details and of the concept would be very much appreciated.

Eric W. Biederman (26):
alpha: Remove unused TASK_GROUP_LEADER
cgroup: Don't open code tasklist_empty()
signal: Do not perform permission checks when sending pdeath_signal
signal: Make group_send_sig_info static
exit: Remove the pointless clearing of SIGPENDING in __exit_signal
rlimit: Remove unnecessary grab of tasklist_lock
pidns: Improve the error handling in alloc_pid
exit: Make the runqueue rcu safe
signal: Don't allow sending SIGKILL or SIGSTOP to init
ptrace: Simplify ptrace_detach & exit_ptrace
wait: Properly implement __WCLONE handling in the presence of exec and ptrace
wait: Directly test for the two cases where wait_task_zombie is called
wait: Remove unused delay_group_leader
wait: Move changing of ptrace from wait_consider_task into wait_task_stopped
wait: Don't delay !ptrace_reparented leaders
exit: Fix reporting a ptraced !reparented leader has exited
exit: Rework the exit states for ptracees
wait: Fix WSTOPPED on a ptraced child
wait: Simpler code for clearing notask_error in wait_consider_task
wait: Don't pass the list to wait_consider_task
wait: Optmize waitpid
exit: Fix auto-wait of ptraced children
signal: Fix SIGCONT before group stop completes.
signal: In ptrace_stop improve identical signal detection
signal: In ptrace_stop use CLD_TRAPPED in all ptrace signals
pidns: Ensure zap_pid_ns_processes always terminates

 arch/alpha/kernel/asm-offsets.c |   1 -
 include/linux/sched.h           |   7 +-
 include/linux/sched/signal.h    |  29 ++--
 include/linux/sched/task.h      |   4 +-
 include/linux/signal.h          |   1 -
 kernel/cgroup/cgroup.c          |   2 +-
 kernel/exit.c                   | 375 ++++++++++++++++------------------------
 kernel/fork.c                   |   3 +-
 kernel/pid.c                    |   8 +-
 kernel/pid_namespace.c          |   2 +-
 kernel/ptrace.c                 |  44 ++---
 kernel/sched/core.c             |   2 +-
 kernel/sched/fair.c             |   2 +-
 kernel/signal.c                 | 122 +++++++------
 kernel/sys.c                    |  13 +-
 15 files changed, 267 insertions(+), 348 deletions(-)

Eric